1. Field of the Invention
The invention generally relates to circuit design and more specifically relates to automated routing of integrated circuits.
2. Description of the Related Art
Integrated circuits have a long history of growing in complexity and scale. While laying out the first inverter or current mirror may not have presented challenges, integrated circuits have grown exponentially in terms of transistors and other circuit elements. With this growth in circuit elements has come similar growth in interconnection between circuit elements. Limited spatial resources available in integrated circuits have ied to tradeoffs between circuit complexity and design time. At times, integrated circuits have been constrained due to overall size of wafers and defect densities, resulting in a practical limit on the number of transistors on an integrated circuit die, and a corresponding limit on design complexity. At other times, integrated circuits have been constrained by routing resources, resulting in fabrication processes using four or more layers of metal to route signal conductors for large and complex digital logic.
Analog circuit designers tend to face additional challenges when designing, placing and routing a circuit for fabrication as an integrated circuit. For example, analog circuits are notoriously sensitive to noise, whereas digital circuits tend to exhibit robust behavior in the face of noise. Analog circuits may be fabricated using a wide range of technologies and a wide range of fabrication factories, resulting in a myriad of options for design constraints arising out of the manufacturing process. For example, conductors in an integrated circuit are typically constrained by a minimum separation distance. Moreover, conductors of all types, but particularly some analog circuit conductors are subject to electromigration problems, thus requiring a minimum height and width to the conductors. The height of a conductor may be process dependent, such that only a minimum height may be specified, with an expectation that the actual height will be greater by an unpredictable amount or percentage. Thus, the width of the conductor may be constrained to be wider than might otherwise be desirable, to allow for conduction of high amperage currents.
Moreover, analog circuits operate as fast as current flows, typically without the benefit of a clock which allows for alignment of signals and margin for error in signal skew. Thus, when an analog circuit includes a differential pair of signals, skew between the signals (such as due to conductor differences or routing differences) may result in improper operation. An amplifier with two differential inputs having unreasonable skew may produce an unacceptable output (incorrect polarity and magnitude for example) due to comparison of two input signals which were skewed. Moreover, transition of the output to an acceptable output may be slow due to slew rate limitations imposed by power supply conductors in a circuit. As such, routing signals in an integrated circuit can be of great importance to the process of designing and fabricating an analog circuit. Long signal conductors will likely have a direct effect on the response time a circuit exhibits, mismatched signal conductors will likely result in unacceptable performance, and inefficient routing may result in overly large (and therefore costly) integrated circuits.